Anti-skid brake control system with feature for assuring derivation of vehicle speed representing value precisely reflecting actual vehicle speed

ABSTRACT

An anti-skid brake control system detects braking condition on at least two wheels for detecting synchronous braking condition in the wheels. Increasing of braking fluid pressure in one wheel cylinder associated with one of the wheels is delayed with respect to the brake fluid pressure increasing timing of the other wheel cylinder associated with the other wheel. By delaying increasing of the braking pressure, the braking fluid pressure in one of the wheel cylinder is maintained lower than that in the other. Therefore, when one of the wheel, for which higher braking pressure is applied through the associated wheel cylinder, the other wheel is maintained at non-locking state. Therefore, the wheel speed closely reflects the actual vehicle speed. This assures acurate brake control in the anti-skid brake control for the automotive vehicle utilizing the vehicle speed representing value derived based on the rotation speed of non-locking wheel.

BACKGROUND OF THE INVENTION

The present invention relates generally to an anti-skid brake controlsystem for an automotive vehicle. More particularly, the inventionrelates generally to an anti-skid brake control system, in which avehicle speed representing value is derived on the basis of a wheelspeed. Further particularly, the invention relates generally to ananti-skid brake control system which prevents all of wheels from lockingsimultaneously and whereby assures derivation of the vehicle speedrepresenting value precisely corresponding to an actual vehicle speedfor assuring accurate brake control even on substantially slippery roadcondition.

DESCRIPTION OF THE PRIOR ART

As is well known, wheel slip control, such as anti-skid brake controland traction control which controls, driving torque on vehicular wheelsfor eliminating wheel-spin, is performed generally based on a wheelslippage. The wheel slippage is generally derived on the basis of adifference between a wheel rotation speed and a vehicle speed. Namely,anti-skid brake control is carried out to maintain wheel slippage around10 to 20% for optimizing vehicular braking characteristics. On the otherhand, in the traction control, a wheel-spinning condition is detected bydetecting wheel speed exceeding a vehicle speed representative value.

Therefore, in order to perform precise wheel slip control, it isessential to detect vehicle speed accurately. Substantially highaccuracy of vehicle speed detection can be made by means of a knownradar systems, such as a dopper radon system. However, such radar systemare expensive facilitating for wheel slip control. Therefore, inpractice, the vehicle speed representing value has to be derived basedon other parameters which can be obtained an inexpensive sensor orsensors.

Conventionally, the vehicle speed representing values have been derivedon the basis of rotation speeds of vehicular wheels. For example, theJapanese Patent Second (examined) Publication (Tokko) Showa No. 41-17082discloses an anti-skid brake control system with derivation of thevehicle speed representing value. In the disclosed procedure, one of thewheels, which is rotating at the highest speed is selected to take therotation speed thereof, this deriving the vehicle speed representingvalue. However, during abrupt deceleration of the vehicle, even thewheel speed of the highest speed wheel will not represent the vehiclespeed. Therefore, in the conventional systems, the vehicle speedrepresenting value may be arithmetically derived by latching a highestwheel speed upon initiation of one cycle of wheel slip control operationand decreasing a constant value at every given interval for derivingassumed vehicle speed representing values to be used during abruptdeceleration.

Difficulty in performing accurate or precise anti-skid brake controlarises when all of wheels effect wheel-locking in order to abruptly dropthe wheel speed. When all of the wheels are locked and skidding, thevehicle speed representing value, derived from on the highest wheelspeed value becomes completely useless, since that value does notrepresents the true vehicle speed. In order to avoid this, there is oneapproach which can be taken in order to provide a back-up value which isto be used as vehicle speed representing value while all the wheels areskidding. In such a system, locking of all the wheels has to be detectedin a precise manner so as to assure switching of the use of the vehiclespeed representing value derived based on the highest wheel speed andthe back-up value. However, difficulty is encountered in detecting whenall of the wheels are locking. Particularly, when the anti-skid controlis utilized in a four wheel drive vehicle and all of the wheels areconnected to the engine, the inertia moment exerted onto each wheelbecomes relatively high to slow-down dropping of the wheel speed.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide ananti-skid brake control system which successfully prevents all of wheelsfrom locking simultaneously.

Another object of the present invention is to provide an anti-skid brakecontrol system which can initiate anti-skid brake control before all ofthe wheels are locked during abrupt braking.

In order to accomplish the aforementioned and other objects, ananti-skid brake control system, according to the present invention,detects braking condition on at least two wheels for detectingsynchronous braking condition in the wheels. Increasing of braking fluidpressure in one wheel cylinder associated with one of the wheels isdelayed with respect to the brake fluid pressure increasing timing ofthe other wheel cylinder associated with the other wheel. By delayingincreasing of the braking pressure, the braking fluid pressure in one ofthe wheel cylinder is maintained lower than that in the other.Therefore, when one of the wheels, for which higher braking pressure isapplied through the associated wheel cylinder, the other wheel ismaintained at non-locking state. Therefore, the wheel speed closelyreflects the actual vehicle speed.

This assures accurate brake control in the anti-skid brake control forthe automotive vehicle utilizing the vehicle speed representing valuederived based on the rotation speed of the non-locking wheel.

According to one aspect of the invention, an anti-skid brake controlsystem for an automotive vehicle comprises a hydraulic brake circuitincluding a plurality of wheel cylinders for establishing brakingpressure for decelerating vehicular wheels, first and second anti-skidbrake control valves, each associated at least one of first and secondwheel cylinders associated with first and second wheels for adjustingbraking pressure therein, each of the anti-skid brake control valvesbeing operable to increase braking pressure in the associated wheelcylinder in a first mode position, and to decrease the braking pressurein the associated wheel cylinder in a second mode position, first sensorfor monitoring rotation speed of the first wheel for producing a firstwheel speed indicative signal, second sensor for monitoring rotationspeed of the second wheel for producing a second wheel speed indicativesignal, first means for deriving first control signal for controllingposition of the first anti-skid brake control valve based on the firstwheel speed indicative signal between the first and second modepositions, second means for deriving second control signal forcontrolling position of the second anti-skid brake control valve basedon the second wheel speed indicative signal between the first and secondmode positions, and third means for detecting synchronous increasing ofbraking pressure in the first and second wheel cylinders to causesynchronous locking of the first and second wheels, and producing athird control signal for providing a given delay time to one of thefirst and second means to operate associated one of the first and secondanti-skid brake control valves into the first mode position so as toincrease braking pressure in the first and second wheel cylinders atdifferent timing.

The anti-skid brake control valves are further operable to hold thebraking pressure in a the first and second wheel cylinders constant inthird mode position. The first and second means alternatively switch themode positions of the first and second anti-skid brake control valvesbetween the first and third mode positions so as to increase the brakingpressure in the first and second wheel cylinders in stepwise fashion.The third means includes first counter for incrementing counter value bya value proportional to a period of time to maintain the first anti-skidbrake control valve at the first mode position and a second counter forincreasing counter value by a value proportional to a period of time tomaintain the second anti-skid brake control valve at the first modeposition, and detector for detecting synchronously increasing of brakingpressure in the first and second wheel cylinders by comparing adifference of the counter values of the first and second counters with agiven value.

The anti-skid brake control system further comprises first arithmeticmeans for deriving first wheel acceleration based on the first wheelspeed indicative signal to produce a first wheel acceleration indicativesignal, a second arithmetic circuit for deriving second wheelacceleration based on the second wheel speed indicative signal value toproduce a second wheel acceleration indicative signal, a first peak holdcircuit for holding peak value of the first wheel accelerationindicative signal in each skid cycle, and a second peak hold circuit forholding peak value of the second wheel speed indicative signal in eachskid cycle. The third means further includes a first comparator forcomparing the first counter value with a first reference value derivedbased on the peak value for producing a first comparator signal when thefirst counter value is greater than or equal to the first referencevalue, and a second comparator for comparing the second counter valuewith a second reference value derived based on the peak value forproducing a second comparator signal when the second counter value isgreater than or equal to the second reference value, and the third meansis responsive to the first and second comparator signals under thepresence of the detector signal to control operation of the first andsecond anti-skid brake control valve with the delay time.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIGS. 1(A) and 1(B) are block diagram of an overall wheel slip controlsystem, to which the preferred embodiment of a vehicle speedrepresenting value derivation system according to the invention, isapplied;

FIG. 2 is a circuit diagram of a peak detecting circuit in the wheelspeed representing value derivation circuit of FIG. 1;

FIG. 3 is a chart showing manner of detection of the wheel speed peak tobe carried out by the peak detecting circuit of FIG. 2;

FIG. 4 is a circuit diagram of a variable timer circuit employed in thewheel slip control system of FIG. 1;

FIG. 5 is a chart showing operation of the variable timer circuit ofFIG. 4;

FIG. 6 is a block diagram of a wheel speed drop rate detecting circuitin the preferred embodiment of the anti-skid brake control system ofFIG. 1;

FIG. 7 is a timing charg showing operation of the wheel speed drop ratedetecting circuit of FIG. 6;

FIG. 8 is a block diagram of the preferred embodiment of the vehiclespeed representing value derivation circuit in the wheel slip controlsystem of FIG. 1;

FIG. 9 is a timing chart showing operation of the vehicle speedrepresenting value derivation circuit of FIG. 6;

FIG. 10 is a timing chart showing operation of the anti-skid brakecontrol to be performed by the wheel slip control system of FIG. 1 forcontrolling braking pressure for rear wheels;

FIG. 11 is a timing chart showing operation of the anti-skid brakecontrol to be performed by the wheel slip control system of FIG. 1 forcontrolling braking pressure for front wheels; and

FIG. 12 is a block diagram of another embodiment of the preferredembodiment of the vehicle speed representing value derivation circuit ofthe invention..

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, particularly to FIG. 1, a wheel slipcontrol system is designed for controlling wheel slip at respectivefront and rear wheels 1, 2, 3 and 4. Each front and rear wheels 1, 2, 3and 4 are co-operated with wheel cylinders 1a, 2a, 3a and 4a so as to beapply braking force therethrough. On the other hand, the rear wheels 3and 4 are connected to an automotive engine through a power trainconstituted by a transmission 6, a propeller shaft 7, a differentialgear unit 8 and drive shafts 9 and 10.

In the shown embodiment, the wheel slip control system controls brakingforce to be exerted to each wheel cylinders for performing anti-skidbrake control for preventing the wheels from skidding. On the otherhand, the wheel slip control system controls engine output torque ordriving torque distribution to the driven rear wheels 3 and 4 so as toprevent the wheels from causing wheel-spin and to provide bettertire/road traction. The engine output torque control or powerdistribution control may be performed by adjusting throttle valveangular position by means of a throttle valve servo systems, such asthat disclosed in British Patent First Publications Nos. 2,154,763 and2,154,765. The disclosure of the above-identified British Patent FirstPublications are herein incorporated by reference for the sake ofdisclosure. Furthermore, traction control systems for adjusting drivingtorque to be exerted on the driven wheels of the vehicle have beendisclosed in the co-pending U.S. patent application Ser. No. 903,474,filed on Sept. 4, 1986, assigned to the common assignee to the presentinvention. The disclosure of the above-identified co-pending U.S. PatentApplication is also herein incorporated by reference for the sake ofdisclosure.

In the shown embodiment, a hydraulic brake system for applying brakingfluid pressure to respective wheel cylinders 1a, 2a, 3a and 4a includesseparate two hydraulic circuits 12 and 14. One of the circuit 12connects one of the outlet ports of a master cylinder 11 to front-rightwheel cylinder 1a which is associated with the front-right wheel 1. Thecircuit 12 includes a branch circuit 13 connecting one of the outletport to the front-left wheel cylinder 2a of the front-left wheel 2. Theother outlet port of the master cylinder 11 is connected to therear-right wheel cylinder 3a and, through a branch circuit 15, to therear-left wheel cylinder 4a. As is well known, the master cylinder 11 ismechanically coupled with a brake pedal 16 for building-up braking fluidpressure depending upon the magnitude of depression of the brake pedal.The braking fluid pressure built up in the master cylinder 11 isdistributed to respective wheel cylinders 1a, 2a, 3a, and 4a through thecircuits 12, 13, 14 and 15 for exerting braking force to respectivelyassociated wheels 1, 2, 3 and 4.

Anti-skid brake control valves assemblies 17a and 17b and 17c aredisposed in the circuits 12, 13 and 14. As will be seen from FIG. 1(A).1(B) and I(C). the anti-skid brake control valve assembly 17c ispositioned upstream of the junction between the circuits 14 and 15.Therefore, braking fluid pressure to be exerted to the rear wheelcylinders 3a and 4a is commonly controlled by means of the anti-skidbrake control valve assembly 17c. On the other hand, the anti-skid brakecontrol valves 17a and 17b are respectively disposed in the circuits 12and 13 at the positions downstream of the junction of the circuits 12and 13. Therefore, the valves 17a and 17b controls braking fluidpressures at associated one of the wheel cylinders 1a and 2a,independently of each other.

The anti-skid brake control valve assemblies 17a, 17b and 17c havemutually identical constructions to each other. Therefore, it would notbe necessary to give a detailed description of the constructions for thevalve assemblies 17a, 17b and 17c, respectively. In order to avoidredundant recitation and to avoid confusion in understanding theinvention, the construction of the anti-skid brake control valveassembly 17a will only be described herebelow. The constructions of thevalve assemblies 17b and 17c should be understood as identical to thatof the valve assembly 17a set out below.

The anti-skid brake control valve assembly 17a includes an inlet (EV)valve 19a, outlet (AV) valve 20a, a fluid pump 21a, accumulator 22a andone-way check valve 23a. The inlet valve 19a has an inlet port connectedto the associated outlet port of the master cylinder 11 via thehydraulic circuit 12 and an outlet port connected to the front-rightwheel cylinder 1a. On the other hand, the AV valve 20a has an inlet portconnected to the wheel cylinder 1a and an outlet port connected to theaccumulator 22a. The accumulator 22a is further connected to thehydraulic circuit 12 via the fluid pump 21a and the one-way check valve23a for feeding back excessive fluid pressure therethrough. The fluidpump 23a is designed to draw the pressurized fluid in the associatedwheel cylinder 1a to quickly reduce the braking pressure in the wheelcylinder while the anti-skid brake control is performed.

The EV valve 19a valve position is controlled by an inlet control signalEV₁ from a control unit 18 between closed position and open position.Similarly, the AV valve 20a valve position is controlled by an outletcontrol signal AV₁ from the control unit 18 between a closed positionand open position. Furthermore, the fluid pump 21a is connected to thecontrol unit 18 to receive a pump drive signal MR its operation iscontrolled between driving condition and resting condition.

In the preferred embodiment, the EV valve 19a is in the open positionwhile the inlet control signal EV₁ is LOW level. On the other hand, theAV valve 20a is in the closed position while the outlet control signalAV₁ is LOW level. The anti-skid brake control valve assembly 17a isoperable in APPLICATION mode. RELEASE mode and HOLD mode. In theAPPLICATION mode, the inlet control signal EV₁ is held LOW to maintainthe EV valve 19a in open position. At the same time, the outlet controlvalve AV₁ is also held LOW to maintain the AV valve 20a in the closedposition. Therefore, the outlet port of the master cylinder 11 isconnected to the wheel cylinder 1a through the hydraulic circuit 12 andthe EV valve 19a. Therefore, the braking fluid pressure in the wheelcylinder 1a is increased proportionally to that built-up in the mastercylinder. On the other hand, in the RELEASE mode, the inlet controlsignal EV₁ is switched into HIGH level to operate the EV valve 19a atthe closed position. This shuts off the fluid communication between theoutlet port of the master cylinder 11 and the wheel cylinder 1atherethrough. On the other hand, at this time, the outlet control signalAV₁ also becomes HIGH level to open the AV valve 20a. As a result, fluidcommunication between the wheel cylinder 1a and the accumulator 22a isestablished. At the same time, the pump drive signal MR turns into HIGHto drive the fluid pump 21. Therefore, the fluid pressure in theaccumulator 22a becomes lower than that in the wheel cylinder 1a.Therefore, the braking fluid in the wheel cylinder 1a is drawn into thepressure accumulator.

On the other hand, in the HOLD mode, the inlet control signal EV₁ is setHIGH level to close the EV valve 19a and the outlet control signal AV₁is set LOW level to maintain the AV valve 20a at closed position.Therefore, the wheel cylinder 1a is blocked from fluid communicationwith the master cylinder 11 and the accumulator 22a. Therefore, thefluid pressure in the wheel cylinder 1a is held constant. Therelationship of the inlet and outlet control signals EV₁ and AV₁ and thepump drive signal MR will be clearly seen from the following table:

                  TABLE                                                           ______________________________________                                        APPLICation        RELEASE   HOLD                                             ______________________________________                                        EV.sub.1                                                                             LOW             HIGH      HIGH                                         AV.sub.1                                                                             LOW             LOW       HIGH                                         MR     --              --        HIGH                                         ______________________________________                                    

The control unit 18 is connected to a wheel speed sensor 26a forreceiving pulse signals having frequency proportional to the rotationspeed of the associated wheel 1. In practice, the wheel speed sensor 26comprises a sensor rotor adapted to rotate with the vehicle wheel and asensor assembly fixedly secured to the shim portion of the knucklespindle. The sensor rotor is fixedly secured to a wheel hub for rotationwith the vehicle wheel. The sensor rotor may be formed with a pluralityof sensor teeth at regular angular intervals. The width of the teeth andthe grooves therebetween are preferably equal to each other and define aunit angle of wheel rotation. The sensor assembly comprises a magneticcore aligned with north pole rear the sensor rotor and south pole distalfrom the sensor rotor. A metal element with a smaller diameter sectionis attached to the end of the magnetic core near the sensor rotor. Thefree end of the metal element faces the sensor teeth. An electromagneticcoil encircles the smaller diameter section of the metal element. Theelectromagnetic coil is adapted to detect a variation in the magneticfield generated by the magnetic core to produce an alternating-currentsensor signal. Namely, the metal element and the magnetic core form akind of proximity switch which adjusts the magnitude of the magneticfield depending upon the distance between the free end of the metalelement and the sensor rotor surface. Thus, the intensity of themagnetic field fluctuates in relation to the passage of the sensor teethand accordingly in relation to the angular velocity of the wheel.

Such a wheel speed sensor has been disclosed in U.S. Pat. No. 4,597,052,issued on June 24, 1986, for example. The disclosure of the U.S. Pat.No. 4,597,052 is herein incorporated by reference for the sake ofdisclosure.

The control unit 18 has a controller circuit section 18a for performingwheel slip control for the right-front wheel 1. Similarly, the controlunit 18 has controller circuit sections 18b and 18c respectively adaptedto perform wheel slip control for respectively associated left-frontwheel 2 and rear wheels 3 and 4. The controller circuit section 18areceives the wheel speed indicative pulse signal from the wheel speedsensor 26a. Similarly, the controller circuit section 18b receives awheel speed indicative pulse signal from a wheel speed sensor 26b whichmonitors rotation speed of the left-front wheel 2. A wheel speed sensor26c is connected to the controller circuit section 18c for inputtingwheel speed indicative pulse signal, which monitors average rotationspeed of the rear wheels 3 and 4. In order to monitor the average speedof the rear wheels, the wheel speed sensor 26c is coupled with thepropeller shaft 7 for monitoring rotation speed thereof as the averationrotation speed of the rear wheels. As the anti-skid brake control valves17a, 17b and 17c, the controller circuit sections 18a, 18b and 18c areessentially the identical circuit constructions to each other.Therefore, the following discussion will be given only for thecontroller circuit section 18a. The corresponding circuit elements inthe controller circuit sections 18b and 18c are represented by the samereference numerals with different suffixes which identifies the sectionto include the circuit elements.

As will be seen from FIG. 1, the controller circuit section 18a includesa wheel speed derivation circuit 31a and a wheel acceleration derivationcircuit 32a. The wheel speed derivation circuit 31a receives the wheelspeed indicative pulse signal from the wheel speed sensor 26a. Based onthe frequency or pulse period of the wheel speed indicative pulse signalof the wheel speed sensor 26a and rotation radius of the right-frontwheel, the wheel speed derivation circuit 31a derives an angularvelocity of the right-front wheel and derives an instantaneous wheelspeed Vw₁ of the right-front wheel 1. The wheel speed derivation circuit31a produces a wheel speed signal indicative of the derived wheel speedVw₁. The wheel acceleration derivation circuit 32a receives the wheelspeed signal from the wheel speed derivation circuit 31a. The wheelacceleration derivation circuit 32a derives wheel acceleration αw₁ andproduces a wheel acceleration indicative signal. Derivation of the wheelacceleration αw₁ based on the variation of wheel speed Vw₁ indicated inthe wheel speed signal, can be performed by differentiating thedifferences of the wheel speeds input at different and consecutivetiming. Otherwise, the wheel acceleration may be derived directly fromthe wheel speed indicative pulse signal from the wheel speed sensor 26ain a manner disclosed in the aforementioned U.S. Pat. No. 4,597,052.

The wheel acceleration indicative signal of the wheel accelerationderivation circuit 32a is transmitted to a comparators 33a and 34a. Inorder to receive the wheel acceleration indicative signal, thecomparator 33a is connected to the wheel acceleration derivation circuit32a at an inverting input terminal. On the other hand, the comparator34a is connected to the wheel acceleration derivation circuit 32a at anon-inverting input terminal thereof for receiving the wheelacceleration indicative signal therethrough. The wheel accelerationderivation circuit 32a is further connected to a peak detector circuit44a which is designed for detecting a peak value of the wheelacceleration aw₁ and holds the peak value.

The non-inverting input terminal of the comparator 33a is connected to areference signal generator (not shown) for receiving therefrom adeceleration threshold indicative reference signal -b. The decelerationthreshold indicative reference signal has a value representative of apredetermined deceleration threshold to be compared with the wheelacceleration value αw₁. The comparator 33a normally outputs a LOW levelcomparator signal as long as the wheel acceleration αw₁ is maintainedhigher than the deceleration threshold -b. The comparator 33a isresponsive to the wheel acceleration αw₁ dropping across thedeceleration threshold -b to output a HIGH level comparator signal.

The inverting input terminal of the comparator 34a is connected to areference signal generator (not shown) which generates an accelerationthreshold indicative reference signal having a value indicative of apredetermined wheel acceleration threshold +a. The comparator 34a thuscompares the wheel acceleration αw₁ as indicated in the wheelacceleration indicative signal of the wheel acceleration derivationcircuit 32a with the wheel acceleration threshold +a. The comparator 34agenerally outputs a LOW level comparator signal while the wheelacceleration αw₁ is held lower than the wheel acceleration threshold +a.The comparator 34a produces a HIGH level comparator signal when thewheel acceleration αw₁ rises across the wheel acceleration threshold.

Another comparator 35a is provided in the controller circuit section18a. The comparator 35a has an inverting input terminal connected thewheel speed derivation circuit 31a for receiving therefrom the wheelspeed signal. The comparator 35a also has a non-inverting input terminalconnected to a target wheel speed derivation circuit 28a. The targetwheel speed derivation circuit 28a generally derives a target wheelspeed Vλ on the basis of a vehicle speed representing value Vi and adesired optimum wheel slippage for optimizing vehicular brakingefficiency. As is well known, the vehicular braking efficiency becomesmaximum in a wheel slippage range of 10% to 20%. In the practicalembodiment, the target wheel slippage λ is set at 15%. On the otherhand, the vehicle speed representing value Vi is derived on the basis ofan instantaneous wheel speed Vw at the beginning of each cycle ofanti-skid brake control. The procedure of derivation of the vehiclespeed representing value Vi discussed later. The target wheel speed Vλis derived at a value of 85% of the vehicle speed representing value Vito indicate 15% of wheel slippage. The comparator 35a maintains a LOWlevel comparator signal while the wheel speed Vw is maintained higherthan the target wheel speed Vλ. On the other hand, the comparator signallevel of the comparator 35a turns HIGH when the wheel speed Vw dropsacross the target wheel speed Vλ.

The comparator signals of the comparators 33a and 34a and 35a are fed toinput terminals of an OR gate 36. The comparator 34a and 35a are alsoconnected to an AND gate 38 to feed the comparator signals. The AND gate38a has an inverting input terminal connected to the comparator 35a toreceive therefrom the comparator signal. The comparator 35a is furtherconnected to a variable timer circuit 42a which will be described later.

The output terminal of the OR gate 36a is connected to one inputterminal of an OR gate 40a. The other input terminal of the OR gate 40ais connected to an AND gate 41a. The AND gate 41a has one input terminalconnected to the variable timer 42a. Another input terminal of the ANDgate 41a is connected to the output terminal of an OR gate 90a. The ORgate 90a has one input terminal connected to a timer 91a of asynchronous locking detector circuit, which timer 91a serves as delaycircuit for providing delay to operate the anti-skid brake control valve17a into the APPLICATION mode. Operation of the timer 91 and theassociated circuit thereof will be described in detail later. The otherinput terminal of the OR gate 90a is connected to an oscillator 43awhich serves as a clock generator. The other input terminal of the ANDgate 41a is connected to a retriggerable timer circuit 30 which isdesigned for producing a pump drive signal MR for driving the fluid pump21a, 21b and 21c by applying the pump drive signal MR to pump-drivemotor 24 through a switching transistor 25.

A vehicle speed representing value derivation circuit 27a is connectedto the wheel speed derivation circuit 31a to receive therefrom the wheelspeed signal from time-to-time. The vehicle speed representing valuederivation circuit 27a is also connected to the retriggerable timercircuit 30. The vehicle speed representing value derivation circuit 27ais designed to latch an instantaneous wheel speed Vw as an initialvehicle speed representing value Vi₁ in response to the leading edge ofa HIGH level timer signal serving as the pump drive signal MR. Thevehicle speed representing value derivation circuit 27a derives thevehicle speed representing value Vi₁ on the basis of the initial vehiclespeed representing value corresponding to the latched wheel speed valueVw₁ as set forth above.

The vehicle speed representing value derivation circuit 27a isassociated with other vehicle speed representing value derivationcircuits 27b and 27c which derive the vehicle speed representing valuesVi₂ and Vi₃ with respect to respectively associated left-front wheel 2and rear wheels 3 and 4, in order to form the preferred embodiment of avehicle speed representing value derivation system 27. The vehicle speedrepresenting value derivation system 27 also includes a select-LOWswitch 58 having three terminals respectively connected to the vehiclespeed representing value derivation circuits 27a, 27b and 27c. Theselect-LOW switch 58 selects the smallest value among three vehiclespeed representing values Vi₁, Vi₂ and Vi₂ from respective vehicle speedrepresenting value derivation circuits 27a, 27b and 27c and outputs theselected value as a common vehicle speed representing value Vi. Thecommon vehicle speed representing value Vi is transferred to respectivetarget wheel speed derivation circuits 28a, 28b and 28c in thecontroller circuit sections 18a, 18b and 18c so that the target wheelderivation circuits will be able to derive the target wheel speeds Vλ₁,Vλ₂ and Vλ₃ based thereon.

The preferred embodiment of the anti-skid brake control system,according to the present invention is adapted to prevent the frontwheels. i.e. right-front wheel 1 and the left-front wheel 2 fromsynchronously locking. Therefore, the synchronous locking detectorcircuit is provided for detecting synchronous lock at the right- andleft front wheels 1 and 2. The synchronous lock detector circuit is thusassociated with the controller circuit sections 18a and 18b. Thesynchronous lock detector circuit includes counters 94a and 94b. Thecounter 94a is connected to the output terminal of the OR gate 40a atinput terminal. The counter 94b is triggered by LOW level gate signalfrom the OR gate 40a to count-up an internal clock for increasing thecounter value thereof according to the length of period to maintain theanti-skid brake control valve 17a in APPLICATION mode. On the otherhand, the counter 94a is connected to a shot pulse generator 95a toreceive therefrom a rest pulse at its reset input terminal to clear thecounter value in response to the reset pulse. The shot-pulse generator95a is, in turn, connected to the output terminal of the comparator 35ato be triggered by the trailing edge of the HIGH level comparator signaltherefrom. Similarly, the counter 94b is connected to the outputterminal of the OR gate 40b at input terminal. The counter 94b istriggered by LOW level gate signal from the OR gate 40b to count-up aninternal clock for increasing the counter value thereof according to thelength of period to maintain the anti-skid brake control valve 17b inAPPLICATION mode. On the other hand, the counter 94b is connected to ashot pulse generator 95b to receive therefrom a rest pulse at its resetinput terminal to clear the counter value in response to the resetpulse. The shot-pulse generator 95b is, in turn, connected to the outputterminal of the comparator 35b to be triggered by the trailing edge ofthe HIGH -level comparator signal therefrom, connected to the AND gates41a and 41b.

The counter 94a is connected to the non-inverting input terminal of acomparator 93a and a subtracting circuit 100. The inverting inputterminal of the comparator 93a is connected to the peak hold circuit 44athrough a 3/4 multiplier 96a to receive a 3/4 multiplied peak valueindicative signal. Similarly, the counter 94b connected to thenon-inverting input terminal of a comparator 93b and a subtractingcircuit 100. The inverting input terminal of the comparator 93b isconnected to the peak hold circuit 44a through a 3/4 multiplier 96b toreceive a 3/4 multiplied peak value indicative signal.

The output terminal of the comparator 93a is connected to one inputterminal of an AND gate 99a. Another input terminal of the AND gate 99ais connected to a wheel speed drop-rate detector circuit 102a. The wheelspeed drop-rate detector circuit 102a is receives the comparators 34aand 35a to receive therefrom the comparator signals. The other inputterminal of the AND gate 99a is connected to the output terminal of acomparator 101 which receives the output of the subtractor circuit 100at its inverting input terminal. A reference value signal is applied tothe non-inverting input terminal of the comparator 101. The outputterminal of the AND gate 99a is connected to one input terminal of anAND gate 92a. Another input terminal of the AND gate 92a is connected tothe output terminal of the timer 91b via an inverter 98a. The otherinput terminal of the AND gate 92a is connected to the output terminalof an AND gate 92b. The AND gate 92a has an output terminal connected tothe timer 91a via an inverter 97.

The wheel speed drop-rate detector circuit 102a is designed fordetecting variation rate of the rotation speed of the right-front wheel1 and compares the derived variation rate with a predetermined value toproduce a HIGH level detector signal c₃ when the variation rate issmaller than or equal to the predetermined value.

The output terminal of the comparator 93b is connected to one inputterminal of an AND gate 99b. Another input terminal of the AND gate 99bis connected to a wheel speed drop-rate detector circuit 102b. The wheelspeed drop-rate detector circuit 102b receives the comparators 34b and35b to receive therefrom the comparator signals. The other inputterminal of the AND gate 99b is connected to the output terminal of thecomparator 101. The output terminal of the AND gate 99b is connected toone input terminal of an AND gate 92b. The other input terminal of theAND gate 92b is connected to the output terminal of the timer 91a via aninverter 98b.

FIG. 2 shows detailed construction of the peak detector circuit 44a inthe controller circuit section 18a set forth above. As will be seen fromFIG. 2, the peak hold circuit 44a generally comprises a peak holdcircuit 49a and an analog switch 49b. The peak hold circuit 48 isconsisted of buffer amplifiers 45 and 46, a diode 47 and a capacitor 48.The buffer amplifier 45 of the peak hold circuit 49a is connected to thewheel acceleration derivation circuit 32a to receive therefrom the wheelacceleration indicative signal having a value representative of thewheel acceleration αw₁, at a non-inverted input terminal. An invertinginput terminal of the buffer amplifier 45 is connected to an outputterminal thereof for receiving the amplifier output as feedback input.The buffer amplifier outputs through its output terminal the amplifieroutput indicative of one of the inputs greater than the other. Theamplifier output of the buffer amplifier 45 is fed to a charge/dischargecircuit consisting of the diode 47 and the capacitor 48 and to anon-inverted input terminal of the other buffer amplifier 46. Similarlyto the aforementioned buffer amplifier 45, an inverting input terminalof the buffer amplifier 46 is connected to an output terminal thereof toreceive the amplifier output as feedback input.

The input side of the capacitor 48 is connected to the ground throughthe analog switch 49b. The analog switch 49b is connected to thecomparator 33a. Therefore, the analog switch 49a becomes conductive atevery leading edge of the HIGH level comparator signal from thecomparator 33a for connecting the capacitor 48 to the ground. As aresult, the potential in the capacitor 48 is discharged to the ground.Since the analog switch 49a is held conductive while the comparatorsignal from the comparator 33a is held at HIGH level, the potential inthe capacitor 48 is maintained substantially zero during this period.The analog switch 49a becomes non-conductive in response to the trailingedge of the HIGH level comparator signal from the comparator 33a toblock communication between the capacitor and the ground. As a result,the capacitor 48 starts to be charged by the amplifier output from thebuffer amplifier 45. The potential in the capacitor 48 is increased asincreasing of the wheel acceleration αw₁ and held at the valuecorresponding to the peak value of the wheel acceleration, as shown inFIG. 3. Therefore, the amplifier output from the buffer amplifier 46indicates peak value αw_(max) of the wheel acceleration. As set forthabove, since the potential in the capacitor 48 is discharged every timethe comparator signal rises to HIGH, the peak value αw_(max) output fromthe peak detector circuit 44a represents the peak value of the wheelacceleration αw₁ in each skid cycle, while the anti-skid brake controlis performed.

FIG. 4 shows a detailed construction of the variable timer 42a. Thevariable timer 42a generally comprises a first timer 51 and a secondtimer 52. The first timer 50 has an input port B connected to the outputterminal of the comparator 34a through an inverter 52 to receivetherefrom the inverted comparator signal. The first timer 50 is designedto be triggered in response to the trailing edge of the HIGH level inputat the input port B to output timer signal Q_(A) for a given period oftime T₁. The period T₁ to maintain the timer signal Q_(A) is determinedby a time constant derived from a capacity of a capacitor 54 and aresistance of a variable resistor 55 connected to the terminals T₁ andT₂ of the first timer 50. The variable resistor 55 is connected to theaforementioned peak detector circuit 44a to receive therefrom theamplifier output as a wheel acceleration peak indicative input. Theresistance of the variable resistor 55 is adjusted depending upon thepeak value αw_(max) indicated in the wheel acceleration peak indicativesignal. Therefore, the timer period of the first timer 50 is variable inproportion to the magnitude of the peak value αw_(max).

The timer signal Q_(A) of the first timer 50 is fed to a B input port ofthe second timer 51. The second timer 51 has a time constant circuitconsisted of a capacitor 56 and a variable resistor 57 for providingtimer period T₂. The time constant of the time constant circuit of thecapacitor 56 and the variable resistor 57 is set constant to set thetimer period T₂ constant. Therefore, the second timer 51 is triggered bythe trailing edge of the HIGH level input at the B input terminal. Thesecond timer 51 as triggered outputs HIGH level timer signal for thetimer period determined by the time constant of the time constantcircuit of the capacitor 56 and the variable resistor 57, through Q_(B)output terminal for the given period of time T₂, as shown in FIG. 5.

FIG 6 shows the wheel speed drop-rate detector circuit 102a whichcomprises a flipflop 103, a timer 102, and a comparator 105. Theflipflop 103 has the set terminal connected to the comparator 35a and areset terminal connected to the comparator 34a. The output terminal ofthe flipflop 103 is connected to the timer 104. The flipflop 103 is setby the leading edge of the HIGH level comparator signal of thecomparator 35a to trigger the later by the HIGH level set output of theflipflop 103. On the other hand, the flipflop 103 is reset by thetrailing edge of the HIGH level comparator signal of the comparator 34a.The timer 104 as triggered by the HIGH level set signal from theflipflop 103 integrates a given value to hold the integrated value untilit is reset by subsequently input HIGH level comparator signal of thecomparator 35a. The timer 104 thus outputs a timer signal having a valueindicative of the integrated value. The timer signal value is comparedwith a threshold level which corresponds to 0.5 sec. Therefore, thecomparator 105 outputs a HIGH level comparator signal when the timersignal value is held greater than or equal to the threshold level, i.e.that corresponding to 0.5 sec.

For instance, in the example of FIG. 7, assuming the wheel speed Vw₁drops across the target wheel speed Vλ₁. the comparator signal of thecomparator 35a turns into HIGH level, at a time t₁ '. Therefore, theflipflop 103 is set by the leading edge of the comparator signal of thecomparator 35a. Therefore, the timer 104 is triggered to integrate thegiven value during the period while the HIGH level set signal is inputfrom the flipflop 103. Assuming that the wheel acceleration αw₁increases across the wheel acceleration threshold +a, at a time t₂ ',the comparator signal turns into LOW level. By the trailing edge of theHIGH level comparator signal of the comparator 4a, the flipflop 103 isreset. By this, the timer 104 ceases integration and holds theintegrated value. During this period, the timer 104 successively outputsthe timer signal. When the timer signal value increases across thethreshold level, the comparator signal c₃ of the comparator 105 turnsinto HIGH level. The HIGH level comparator signal is maintained untilthe timer signal drops across the threshold level by resetting the timer104 by the next HIGH level comparator signal from the comparator 35a.

FIG. 8 shows the detailed construction of the vehicle speed representingvalue derivation circuit 27a. As set forth above, the vehicle speedrepresenting value derivation circuit 27a derives a vehicle speedrepresenting value Vi₁ based on the wheel speed Vw₁ as indicated in thewheel speed signal from the wheel speed derivation circuit 31a. Thevehicle speed representing value derivation circuit 27a includescomparators 59 and 60. The comparator 59 has a non-inverting inputterminal connected to the wheel speed derivation circuit 31a. On theother hand, the comparator 60 is connected to the wheel speed derivationcircuit 31a at an inverting input terminal. An inverting input terminalof the comparator 59 is connected to output terminal of the vehiclespeed representing value derivation circuit 27a through which thevehicle speed representing value Vi₁ is output, through an adder 61. Onthe other hand, the non-inverting input terminal of the comparator 60 isconnected to the output terminal of the vehicle speed representing valuederivation circuit 27a through a subtractor 62. The adder 61 is designedto add a given value corresponding to 1 km/h of vehicle speed to thevehicle speed representing value Vi₁ for providing dead band of +1 km/h.The value as the sum of the vehicle speed representing value Vi₁ and thedead band value 1 km/h will be hereafter referred to as higher vehiclespeed reference value. Similarly, the subtractor 672 subtracts a givenvalue corresponding to 1 km/h of the vehicle speed from the vehiclespeed representing value Vi₁ for providing dead band of -1 km/h. Thevalue as the difference of the vehicle speed representing value Vi₁ andthe dead band value -1 km/h will be hereafter referred to as lowervehicle speed reference value. The comparator 59 outputs HIGH levelcomparator signal when the wheel speed vw.sub. 1 is higher than or equalto the higher vehicle speed reference value (Vi₁ +1 km/h). In otherwords, the comparator signal level of the comparator 59 is held LOW aslong as the wheel speed Vw₁ is maintained lower than the lower vehiclespeed reference value (Vi₁ +1 km/h). The comparator 60 outputs HIGHlevel comparator signal when the wheel speed vw₁ is lower than to thelower vehicle speed reference value (Vi₁ -1 km/h). In other words, thecomparator signal level of the comparator 59 is held LOW as long as thewheel speed Vw₁ is maintained higher than or equal to the lower vehiclespeed reference value (Vi₁ -1 km/h).

The output terminals of the comparators 59 and 60 are connected to inputterminals of NOR gate 63 to feed the comparator signals c₁ and c₂thereto. The NOR gate 63 outputs HIGH level gate signal while signallevels of both of the comparator signals c₁ and c₂ are maintained LOW.Namely, the gate signal output from the NOR gate 63 is held LOW whilethe wheel speed Vw₁ is maintained higher than or equal to the vehiclespeed representing value Vi₁ -1 km/h and lower than the higher vehiclespeed reference value (Vi₁ +1 km/h). The gate signal of the NOR gate 63is fed to a timer 64, an OR gate 65 and a shot-pulse generator 66,respectively. The timer 64 is responsive to the trailing edge of theHIGH level NOR gate signal to output a timer signal for a given periodof time T₃. e.g. 0.1 sec. The timer signal is fed to the OR gate 65.

The OR gate 65 thus receives the NOR gate signal at one input terminaland the timer signal from the timer 64 at the other input terminal. AnOR gate signal of the OR gate 65 is transmitted to a gate of an analogswitch 67 as a selector signal S₃. The output terminal of the OR gate 65is also connected to one input terminal of an AND gates 69 and 70 via aninverter 68. The other input terminal of the AND gate 69 is connected tothe output terminal of the comparator 59 to receive therefrom thecomparator signal c₁. Similarly, the other input terminal of the ANDgate 70 is connected to the output terminal of the comparator 60 toreceive the comparator signal c₂ therefrom. Therefore, the gate signalS₂ of the AND gate 69 becomes HIGH while the comparator signal c₁ ismaintained at HIGH level and the NOR gate signal is held LOW. The gatesignal S₂ serves as a selector signal. On the other hand, the gatesignal S₄ of the AND gate 70 becomes HIGH level while the comparatorsignal c₂ is maintained HIGH and the NOR gate signal is held LOW. Thisgate signal S₄ also serves as selector signal. The AND gates 69 and 70are connected to gates of analog switches 71 and 72.

The analog switch 67 is turned ON in response to HIGH level selectorsignal S₃ to drop the supply voltage to an integrator circuit 73 tozero. On the other hand, the analog switch 71 is turned ON in responseto the HIGH level selector signal S₂ to supply a voltage E correspondingto a possible maximum wheel acceleration, e.g. 0.4G, to the integratorcircuit 75. The analog switch 72 is likewise turned ON in response tothe HIGH level selector signal S₄ to supply a voltage corresponding topossible minimum wheel acceleration value. e.g -1.2G, to the integratorcircuit 73.

The integrator circuit 73 has a per se well known construction and isconsisted of an amplifier 74, a capacitor 75 and an analog switch 76.The gate of the analog switch 76 is connected to the shot-pulsegenerator 66 to receive therefrom a shot-pulse which serves as a resetsignal S₁. The integrator 73 is reset by the HIGH level reset signal S₁and is responsive to the trailing edge of the HIGH level reset signal toreset the integrated value. The integrator circuit 73 integrates thesupply voltage E after termination of the HIGH level reset signal S₁ tooutput the integrator signal. The shot-pulse generator 66 is responsiveto an ON-set signal IG of an ignition switch to generate a first shotpulse as the first reset signal for resetting the integrator circuit 73.The shot-pulse generator 66 subsequently generates the shot-pulsesserving as the reset signal S₁ at every leading edge of the HIGH levelNOR gate signal. As set forth above, since the NOR gate signal becomesHIGH when the wheel speed Vw₁ satisfies (Vi₁ -1 km/h) Vw₁ (Vi.sub. 1 +1km/h), the integrated value of the integrator 73 is reset everyoccurrence of the wheel speed Vw₁ in the aforementioned range. The resetsignal S₁ of the shot-pulse generator 66 is also supplied to a samplehold circuit 77. The sample hold circuit 77 comprises buffer amplifiers78 and 79, a capacitor 80 and an analog switch 81. The analog switch 81is connected to the shot-pulse generator 66 to receive the reset signalS₁ at the gate thereof to be turned ON. The sample hold circuit 77 isresponsive to turning ON of the analog switch 81 to reset the held wheelspeed value. The sample hold circuit 77 in absence of the reset signalS₁ from the shot-pulse generator 66, samples and holds the instantaneouswheel speed value Vw₁ at the occurrence of the reset signal as a samplevalue Vs. The sample hold circuit 77 outputs a sample/hold signal havinga value indicative of the sample value Vs to an adder 82. The adderreceives the sample/hold signal from the sample hold circuit 77 andintegrator signal from the integrator 73. As will be appreciated, theintegrator signal has a value indicative of an integrated value Ve##EQU1## Therefore, the adder 82 adds the integrated value Ve to thesample value Vs to derive the vehicle speed representing value Vi₁. Theoutput terminal of the adder 82 is connected to a switching circuit 83.The switching circuit 83 is also directly connected to the wheel speedderivation circuit 31a to be input the wheel speed signal. On the otherhand, the switching circuit 83 also connected to an AND gate 84. The ANDgate 84 has one input terminal connected to a retriggerable timer 30 toreceive therefrom the pump drive signal MR. The other input terminal ofthe AND gate 84 is connected to the output terminal of the comparator59. The AND gate 84 controls the switch position of the switchingcircuit 83 to selectively connect the wheel speed derivation circuit 31aor the adder 82 to the output terminal of the vehicle speed representingvalue derivation circuit 27a.

Namely, the gate signal of the AND gate is normally maintained LOW leveldue to absence of the HIGH level pump drive signal MR. The gate signalof the AND gate is also held LOW level while the wheel acceleration isnegative or that the wheel speed Vw₁ is lower than the Vi₁ +1 km/h valueas compared in the comparator 59. While the gate signal is held LOW, theswitching circuit 85 is held at a first switch position where the adder82 is connected to the output terminal of the vehicle speed representingvalue derivation circuit therethrough. On the other hand, when the HIGHlevel pump drive signal MR and the HIGH level comparator signal of thecomparator 59 are both input to the AND gate, the gate signal of the ANDgate 84 turns HIGH to switch the switching circuit 83 to a second switchposition where the wheel speed derivation circuit 31a is directlyconnected to the output terminal of the vehicle speed representing valuederivation circuit 27a.

Operation of the vehicle speed representing derivation circuit 27a willbe described herebelow with reference to FIG. 9. In FIG. 9, theoperation of the vehicle speed representing value derivation circuit 27ais described in a condition where the gate signal level of the AND gate84 is maintained LOW level due to absence of the HIGH level pump signalMR or the comparator signal c₁ from the comparator 59 is held LOW. Atthis condition, by the LOW level gate signal of the AND gate 84, theswitching circuit 83 is switched at a switch position where the adder 82is connected to the output terminal of the vehicle speed representingvalue derivation circuit.

In the process of FIG. 9, the engine is started up at a time t₀. Inresponse thereto, the On-set signal IG is input to the shop-pulsegenerator 66. Therefore, the shot pulse s₁ is output at the time t₀ fromthe shot-pulse generator 66. With the shot pulse at the time t₀, thesample hold circuit 77 is reset. The sample/hold circuit 77 as reset bythe shot pulse s₁ at the time t₀, samples and holds the wheel speedsignal value Vw₁ as sample value Vs. Therefore, after the time t₀. theheld sample value Vs is output from the sample hold circuit 77 as aninitial vehicle speed representing value. At the same time, i.e. at thetime t₀, the integrator circuit 73 is reset by the reset signal s₁.Therefore, the value Ve of the integrator signal of the integratorcircuit 73 drops to zero. As a result, the output value Vi₁ from theadder 82 becomes that equal to the held initial vehicle speedrepresenting value Vs, as shown by broken line in FIG. 9.

At this time, the comparator signals c₁ and c₂ of the comparators 59 and60 are maintained LOW level. Therefore, the NOR gate signal of the NORgate 63 is maintained HIGH level. Therefore. the gate signal level ofthe OR gate 65 is maintained HIGH level and the gate signal is fed tothe analog switch 67 as the select signal S₃. Therefore, the analogswitch 67 is turned ON by the HIGH level gate signal from the OR gate65. On the other hand, the HIGH level gate signal of the OR gate 65 isfed to the AND gates 69 and 70 through the inverter 68. Therefore, theselect signals S₂ and S₄ of the AND gates 69 and 70 are held LOW tomaintain the analog switches 71 and 72 at OFF position. Since the analogswitch 67 serves to maintain the input level at the inverting inputterminal of the comparator 74 in the integrator circuit 73 zero, theintegrated value of the integrator circuit 73 is maintained zero. As aconsequence, the output value of the adder 82 as the vehicle speedrepresenting value Vi is maintained at the equal value to the initialvehicle speed representing value as the sample value Vs.

After the vehicle starts running, the wheel speed Vw becomes greaterthan or equal to Vi₁ +1 km/h, at a time t₁. In response to this, thecomparator signal of the comparator 59 turns HIGH level. By the HIGHlevel comparator signal c₁ from the comparator 59, the gate signal ofthe NOR gate 63 turns LOW. At this time, since the timer 64 becomesactive to output HIGH level timer signal for a period of time T₃, thegate signal level of the OR gate 65 is maintained HIGH level for thecorresponding T₃ period. Therefore, the select signal S₃ is maintainedHIGH level and the selector signals S₂ and S₄ are held at LOW level.Therefore, even after the vehicle start running the vehicle speedrepresenting value Vi₁ is held at the equal value to the sample value Vsfor the T₃ period.

After expiration of the T₃ period, at a time t₂, the gate signal of theOR gate 65 turned into LOW level due to termination of the HIGH leveltimer signal from the timer 64. Since the comparator signal c₁ and theinverted gate signal from the OR gate 65 through the inverter 68 areboth becomes HIGH level, the selector signal S₂ of the AND gate 69 turnsHIGH. At the same time, because of LOW level gate signal of the OR gate65 is applied to the gate of the analog switch 67 as the select signalS₃ to turn the latter OFF. At this time, since the comparator signalfrom the comparator 60 is maintained LOW level, the AND gate 70 ismaintained non-conductive to feed the LOW level select signal S₄.Therefore, only analog switch 71 is turned ON to input a valuecorresponding to wheel acceleration magnitude of 0.4G. This value, e.g.0.4G serves for defining inclination of the vehicle speed representingvalue Vi₁. The integrator circuit 73 thus receives the value (0.4G)through the analog switch 71 to output the integrator signal having avalue Ve as set forth above. Therefore, the output value of the adder 82increases from time-to-time as increasing of the integrator signal valueVe.

At a time t₃, the vehicle speed representing value Vi₁ (=Vs+Ve) reachesa value to establish Vw₁ <Vi₁ +1 km/h. Then, the comparator signal c₁turns into LOW level. Therefore, the gate signal level of the NOR gate63 again turns into HIGH level. The shot-pulse generator 66 is triggeredby the leading edge of the HIGH level gate signal of the NOR gate tooutput the shot pulse serving as the reset pulse S₁. Therefore, thesample hold circuit 77 and the integrator circuit 73 are reset. At thesame time, the instantaneous wheel speed Vw₁ at the time t₃ is sampledand held in the sample hold circuit 77 as the renewed sample value Vs.By renewing the sample value Vs, the vehicle speed representing valueVi₁ becomes equal to the instantaneous wheel speed value Vw₁ and thusestablishes Vw₁ ≧Vi+1 km/h. Therefore, the comparator signal c₁ againturns ON at the time t₃. Similarly to the control behavior at the periodfrom t₁ to t₃, the gate signal of the OR gate 65 is held HIGH for the T₃period by HIGH level timer signal of the timer 64. Similarly, at thetime t₄, the shot-pulse generator 66 is triggered to output the resetsignal S₁ to renew the sample value Vs by the instantaneous wheel speedVi₁ at the time t₄. After the time t₄, the sampled value Vs ismaintained constant for the given period T₃ by the HIGH level timersignal of the timer 64. Before the HIGH level timer signal terminates,the shot pulse as the reset signal is generated by the shot pulsegenerator 66, as shown in FIG. 9. While the interval of the shot pulseof the shot pulse generator 66 is shorter than the timer period T₃, theintegrated value of the integrator 73 is maintained zero. Therefore, theadder outputs the sampled value Vs as the vehicle speed representingvalue Vi₁. After a time t₅, at which the reset signal s₁ is produced forresetting the sample hold circuit 77 and the intergrator 73, the gatesignal of the OR gate turns LOW at a time t₆ after expiration of thetimer period T₃. During the period between the times t₅ and t₆, thewheel speed Vw₁ drops to be lower than (Vi₁ -1 km/h). Since the wheelspeed Vw₁ is maintained lower than the value (Vi₁ -1 km/h) at the timet₆, the comparator signal c₁ of the comparator 59 is maintained LOW andthe comparator signal c₂ of the comparator 60 is maintained HIGH.Therefore, the analog switch 71 is held OFF and the analog switch 72 isturned ON. Therefore, a value corresponding to a predetermineddeceleration magnitude of -1.2G is input through the analog switch 72.Consequently, the integrated value in the integrator 73 becomesnegative. The negative integrated value is summed with the sample valueVs which corresponds to the instantaneous wheel speed Vw₁ at a time t₅to gradually reduce the value of the vehicle speed representing valueVi₁. At a time t₇, the wheel speed Vw₁ is increased across the value(Vi₁ +1 km/h). As a result, the reset signal s₁ is generated by the shotpulse generator 66. Then sample hold circuit 77 and the integrator 73are thus reset.

At a time t₈, braking operation is initiated to abruptly decelerate thevehicle. As a result, the wheel speed Vw₁ drops across the value of (Vi₁-1 km/h). By this, the comparator signal c₂ of the comparator rises toHIGH level to destroy the NOR condition at the NOR gate 63. Therefore,the NOR gate signal of the NOR gate turns into LOW level. The timer 64is thus triggered by the trailing edge of the HIGH level NOR gate signalto output HIGH level timer signal for the timer period T₃. Afterexpiration of the timer period T₃, the value corresponding to thedeceleration magnitude of -1.2G is input to the integrator. Therefore,the vehicle speed representing value Vi₁ is gradually decreased.

FIG. 10 shows a timing chart of the operation of the controller circuitsection 18c in anti-skid brake control for the rear wheels.

It is assumed that vehicular brake is applied for deceleration of thevehicle at a time t₁₀, hydraulic braking pressure is built up anddistributed to all of the wheel cylinders 1a, 2a, 3a and 4a in common.According to the increasing of the braking pressure in the wheelcylinders 3a and 4a, the rear wheels 3 and 4 are decelerated.

At a time t₁₁, the wheel acceleration aw₃ decreases across the wheeldeceleration threshold -b. This results in initiation of anti-skid brakecontrol. Namely, since the wheel acceleration aw₃ becomes smaller thanthe wheel deceleration threshold -b, the comparator signal of thecomparator 33c turns into HIGH level. This turns the gate signals of theOR gates 36c, 40c and 40d into HIGH level. Therefore, the inlet controlsignal EV₃ output through the amplifier 37c turns HIGH level. Therefore,both of the EV valves 19c and 19d are shut at the time t₁₁. At thistime, since the output of the AND gate 38c is held LOW level, the outletcontrol signal AV₃ is held LOW. Therefore, the AV valves 20c and 20d arealso maintained at closed position. Therefore, the anti-skid brakecontrol valve assemblies 7c and 17d are placed in HOLD mode position.The braking pressure in the wheel cylinders 3a and 4a are thus heldconstant at the pressure level at the time t₁₁.

For the initial cycle of the anti-skid brake control, the vehicle speedrepresenting value Vi is derived by the vehicle speed representing valuederivation system 27. Based on the common vehicle speed representingvalue Vi, the target wheel speed Vλ₃ is derived at a value 85% of Vi. Asdescribed above, since the vehicle speed representing value decreasesaccording to the integrated value of the integrator 73, the target wheelspeed Vλ₃ decreases continuously. At a time t₁₂, the lower rear wheelspeed Vw_(R) decelerated across the target wheel speed Vλ₃. Then, thecomparator signal of the comparator 35c turns into HIGH level. At thistime, since the comparator signal of the comparator 34c is held LOW toapply HIGH level input to the AND gate 38c through the inverting inputterminal, AND condition is established at the AND gate 38c. Therefore,the gate signal of the AND gate 38c turns into HIGH level to cause HIGHlevel outlet control signal AV₃. This operates the anti-skid brakecontrol valve assemblies 17c and 17d into the RELEASE mode position.Therefore, the braking pressure in the wheel cylinders 3a and 4a arereleased by feeding back the pressurized braking fluid to the pressureaccumulators 22c and 22d. At the same time, by the HIGH level outletcontrol signal AV₃. the retriggerable timer 30 is triggered to startoutputting of the pump drive signal MR. Therefore, the pump motor 24 isstarted to drive the fluid pumps 21c and 21d.

By decreasing the braking pressure in the wheel cylinders 3a and 4a, thewheel speed Vw₃ are resumed and therefore wheel acceleration aw₃ isincreased. The wheel acceleration αw₃ increases across the wheeldeceleration threshold -b at a time t₁₃. Therefore, the comparatorsignal of the comparator 33c turns into LOW level at the time t₁₃.However, at this time, since the HIGH level comparator signal is inputto the OR gate 36 is maintained HIGH level. Therefore, the gate signalof the OR gate 40c is held HIGH to maintain the inlet control signal EV₃at HIGH level. Therefore, the anti-skid control valve assemblies 17c and17d are maintained at the RELEASE mode, at the time t₁₃. Thus, the wheelspeeds Vw₃ are continued to increase.

Accordingly, the wheel acceleration αw₃ increases. At a time ₁₄, thewheel acceleration αw₃ increases across the wheel acceleration threshold+a. This results in HIGH level comparator signal of the comparator 34c.This HIGH level comparator signal of the comparator 34c turns the inputlevel at the inverting input terminal of the AND gate 38c into LOWlevel. Therefore, the AND gate signal of the AND gate 38c turns into LOWlevel. Therefore, the outlet control signal AV₃ turns into LOW level toclose the AV valves 20c and 20d in the anti-skid brake control valveassemblies 17c and 17d. As a result, the anti-skid brake control valveassemblies 17c and 17d are again placed into the HOLD mode to maintainthe braking pressure level constant at the pressure level at the timet₁₄. By holding the braking pressure at decreased level, the wheel speedVw₃ still increases. Also, according to increasing of the wheel speed,wheel acceleration aw₃ increases toward the peak aW_(max). As seen fromFIG. 10, the rear wheel speed Vw₃ increases across the target wheelspeed Vλ₃ at a time t₁₅. This results in LOW level comparator signal ofthe comparator 35c. After this, at a time t₁₆, the wheel accelerationαw₃ drops across the wheel acceleration threshold +a. In response todrop of the wheel acceleration αw₃ across the wheel accelerationthreshold +a, the comparator signal of the comparator 34c turns into LOWlevel. Therefore, all of the inputs to the OR gate 36c becomes LOWlevel. Therefore, the OR gate signal of the OR gate 36c turns into LOWlevel to cause LOW level inlet control signal EV₃ at the time t₁₆. Atthe same time, the variable timer 42c is activated by the trailing edgeof the HIGH level comparator signal of the comparator 34c to output HIGHlevel timer signal for a period T₂ after a delay time T₁ which isvariable depending upon the wheel acceleration peak value as latched bythe peak hold circuit 44c. During the period T₂, the oscillator 43c istriggered to output constant pulse signals. As will be appreciated,while the pulse signal is maintained ON (HIGH) level, AND conditions areestablished in the AND gate 41c to maintain the inlet control signal EV₃at HIGH level. Therefore, during the delay time T₁, the anti-skid brakecontrol valves 17c and 17d are held in APPLICATION mode to increase thebraking pressure. On the other hand, during the period T₂, the operationmode of the anti-skid brake control valves 17c and 17d are alternatedbetween APPLICATION mode and HOLD mode repeatedly.

At a time t₁₇, the wheel acceleration αw₃ drops across the wheeldeceleration threshold -b. Therefore, another cycle of anti-skid brakecontrol is initiated. At the same time, the peak hold circuit 44c isreset by the leading edge of the HIGH level comparator signal of thecomparator 33c. Thereafter, one skid cycle of anti-skid brake controloperation is performed during the period t₁₇ to t₂₂.

As will be appreciated here from, the anti-skid brake control to betaken place for releasing the braking pressure in both of the rear wheelcylinders in response to decreasing of the lower rear wheel speed Vw_(R)across the target wheel speed Vλ₃ in synchronism with each other and atthe same rate. On the other hand, in the APPLICATION mode, the brakingpressures in the rear wheel cylinders are built up at different rate toeach other. As a result, when one of the wheel is decelerated across thetarget wheel speed, the other wheel is rotating at higher speed than thetarget wheel speed. Therefore, it is successfully avoided to causelocking of both wheels at the same time. This assures derivation of thevehicle speed representing value Vi₃ at the value precisely reflectingthe vehicle speed to allow precise control for the brake.

On the other hand, according to the shown embodiment of anti-skid brakecontrol, synchronous lock of the right-front and left-front wheels 3 and4 are prevented by means of the synchronous locking detector circuit.Operation of the anti-skid brake control for the front wheels aredescribed herebelow with reference to FIG. 11.

Similarly to the anti-skid brake control operation for the rear wheels 3and 4, anti-skid brake control for the front wheels 1 and 2 areperformed through the period of t₁₀ to t₁₂. At the time t₁₂, the wheelspeeds Vw₁ and Vw₂ of the right-front wheel 1 and the left-front wheel 2drops across the target wheel speed Vλ₁ and Vλ₂ which are derived on thevehicle speed representing value Vi. Therefore, the comparator signalsof the comparators 35a and 35b turn into HIGH level, at the time t₁₂. Inresponse to the HIGH level comparator signals of the comparators 35a and35b, the outlet control signals AV₁ and AV₂ turn into HIGH level to openthe AV valves 20a and 20b of the anti-skid brake control valves 17a and17b. In response to the leading edge of the HIGH level comparatorsignals of the comparators 35a and 35b, the retriggerable timer 30 istriggered to output the pump drive signal MR. At the same time, theflipflops 103 of the wheel speed drop-rate detector circuit 102a and102b are set by the leading edge of the HIGH level comparator signal 35aand 35b. Therefore, the timer 101 starts integration of the given valueto output the timer signal. The timer value of the timer 101 willbecomes greater than the 0.5 sec representing value so that thecomparator 105 may output the HIGH level comparator signal after 0.5 secof the time t₁₂.

After this the wheel speeds Vw₁ and Vw₂ are resumed by decreasing of thebraking pressures in the wheel cylinders 1a and 2a. At a time t₁₃, thewheel acceleration αw₁ increases across the wheel deceleration threshold-b, and subsequently, at the time t₁₄, the wheel acceleration αw₁increases across the wheel acceleration threshold +a. By increasing thewheel acceleration across the wheel acceleration threshold +a, thecomparator signal levels of the comparators 34a and 34b turn into HIGHlevel. By the HIGH level comparator signals from the comparators 34a and34b, the flip-flops 103 of the wheel speed drop-rate detector circuits102a and 102b are reset to stop integration in the timer 104. At a timet₁₅, the wheel speed Vw₁ increases across the target wheel speed Vλ₁ toturn the comparator signal level of the comparator 35a into LOW level.At this time, the wheel speed Vw₂ of the left-front wheel 2 is stillmaintained lower than the target wheel speed Vλ₂. Therefore, thecomparator signal level of the comparator 35b is maintained HIGH level.Therefore, at the time t₁₅, the anti-skid brake control valve 17a isoperated into HOLD mode, and whereas the anti-skid brake control valve17b is maintained in RELEASE mode to further decreases the brakingpressure in the wheel cylinder 2a. At a time t₁₅, the wheel speed Vw₂decreases across the target wheel speed Vλ₂. Therefore, the comparatorsignal of the comparator 35b turns into LOW level to operate theanti-skid brake control valve 17b into the HOLD MODE.

Therefore, as will be appreciated, at the time t₁₅, the braking pressurelevel in the wheel cylinder 2a is lower than that in the wheel cylinder1a.

After the time t₁₅, the wheel acceleration αw₁ reaches the peak. Thepeak value is held by the peak hold circuits 44a and 44b. Therefore, theinput level at the inverting input terminals of the comparators 93a and93b are increased to 3/4 of the held peak value αw_(max).

As seen from FIG. 11, the peak value αw_(max) as held in the peak holdcircuits 44a and 44b are different from each other. Namely, in the shownexample, the peak value αw_(max) held in the peak hold circuit 44a issmaller than that of the peak hold circuit 44b.

At a time t₁₆, the wheel acceleration αw₁ decreases across the wheelacceleration threshold +a. As a result, the anti-skid brake controlvalves 17a and 17b are operated into APPLICATION mode to increase thebraking pressure in the wheel cylinders 1a and 2a in stepwise fashion byalternating the anti-skid brake control valve positions betweenAPPLICATION mode position and HOLD mode position. During this period,the LOW level OR gates signals of the OR gates 40a and 40b are appliedto the input terminal of the counters 94a and 94b. Therefore, thecounters 94a and 94b integrate a predetermined unit value to producecounter signals. The counter values of the counters 94a and 94b arecompared with the 3/4 of the peak values αw_(max) in the comparators 93aand 93b. The counter values of the counters 94a and 94b, reaches the3/4×αw_(max) values at a time t₁₆. Therefore, the comparator signals ofthe comparators 93a and 93b turns into HIGH level.

While the difference of the counter values of the counters 94a and 94bas derived by the subtractor 100 is held smaller than a given value, thecomparator 101 maintains HIGH level comparator signal. Therefore, thegate signal of the AND gates 99a and 99b turns into HIGH level. At thetime t₁₆, the inputs for the AND gate 92a from the inverters 97 and 98aare held HIGH level and the input for the AND gate 92b from the inverter98b id held HIGH level. However, since the inverted input from theinverter 97 turns LOW level, the AND gate 92a is held the gate signallevel at LOW level. Therefore, only timer 91b is triggered to output theHIGH level timer signal for the given period of time ΔT. The HIGH leveltimer signal is fed to the input terminal of the OR gate 90b to maintainthe gate signal og the OR gate 90b at HIGH level. As a result, the inletcontrol signal EV₂ for the anti-skid brake control valve 17b ismaintained HIGH level.

On the other hand, since the gate signal of the AND gate 92a ismaintained at LOW level as set forth above, the timer signal of thetimer 91a is maintained LOW level to allow altering of the gate signalof the AND gate 41a between HIGH and LOW levels to alternate the inletcontrol signal between HIGH and LOW levels to increase the brakingpressure in the wheel cylinder 1a in stepwise fashion.

Therefore, after the time t_(16a), the braking pressure in the wheelcylinder 1 is continued to increase and the braking pressure in thewheel cylinder 2 is maintained at constant level for the period ΔT.

As will be seen from FIG 11, by continuing increasing of the brakingpressure, the wheel speed Vw₁ is decreased and the wheel accelerationαw₁ drops across the wheel deceleration threshold -b to turn thecomparator signal of the comparator 33a into HIGH level, at a timet_(16b). Subsequently, the wheel speed Vw₁ decreases across the targetwheel speed Vλ₁ to turn the comparator signal of the comparator 35a intoHIGH level. As a result, the counter 94a is reset by the leading edge ofthe HIGH level comparator signal from the comparator 35a. This makes thedifference between the counter values of the counters 94a and 94bgreater than the given value as compared in the comparator 101.Therefore, the comparator signal of the comparator 101 becomes LOWlevel, at the time t_(16b).

After the given period ΔT and at a time t_(16c), the timer signal of thetimer 91b turns LOW level to allow alternating the gate signal level ofthe OR gate 90b between HIGH and LOW levels. Therefore, the fluidpressure in the wheel cylinder 2a increases in stepwise fashion.

In the shown example, the counter value of the counter 94a again reachesthe 3/4×αw_(max) at a time t_(16d). However, at this time, since thedifference of the counter values of the counters 94a and 94b are heldgreater than the given value, the comparator signal of the comparator101 is maintained LOW level. Therefore, the timers 91a and 91b are nottriggered.

Therefore, simultaneous locking of the front wheels can be successfullyavoided.

For instance, FIG. 14 shows a modification of the aforementioned vehiclespeed representing value derivation circuit 27a, 27b, 27c and 27d. Inthis modified embodiment, the wheel acceleration indicative value to beintroduced to the integrator 73 is variable between +1.0G and +0.4Gthrough the analog switch 71. For selectively introducing the +0.4GValue and +10G value, a selector switch 85 is provided. The selectorswitch 85 is normally in a position to pass the +0.4G value to theintegrator 73 and is responsive to HIGH level pump drive signal MR toturn the switch position to pass the +1.0G value. In the modification,the switch 83 is neglected.

What is claimed is:
 1. An anti-skid brake control system for anautomotive vehicle comprising:a hydraulic brake circuit including aplurality of wheel cylinders for establishing braking pressure fordecelerating vehicular wheels; first and second anti-skid brake controlvalves, each associated with at least one of first and second wheelcylinders associated with first and second wheels for adjusting brakingpressure therein, each of said anti-skid brake control valves beingoperable to increase braking pressure in the associated wheel cylinderin a first mode position, and to decrease the braking pressure in theassociated wheel cylinder in a second mode position; first sensor formonitoring rotation speed of said first wheel for producing a firstwheel speed indicative signal; second sensor for monitoring rotationspeed of said second wheel for producing a second wheel speed indicativesignal; first means for deriving a first control signal for controllingsaid first anti-skid brake control valve based on said first wheel speedindicative signal between said first and second mode positions; secondmeans for deriving a second control signal for controlling said secondanti-skid brake control valve based on said second wheel speedindicative signal between said first and second mode positions; andthird means for detecting synchronous increasing of braking pressure insaid first and second wheel cylinders to cause synchronous locking ofsaid first and second wheels, and producing a third control signal forproviding a given delay time to one of said first and second means so asto operate an associated one of said first and second anti-skid brakecontrol valves into said first mode position so as to increase brakingpressure in said first and second wheel cylinders at different timingsand thus preventing synchronous locking of said first and second wheels.2. An anti-skid brake control system as set forth in claim 1, whereinsaid anti-skid brake control valves are further operable to hold thebraking pressure in said first and second wheel cylinders constant athird mode position.
 3. An anti-skid brake control system as set forthin claim 2, wherein said first and second means alternatively switch themode positions of said first and second anti-skid brake control valvesbetween said first and third mode positions so as to increase saidbraking pressure in said first and second wheel cylinders in a stepwisefashion.
 4. An anti-skid brake control system as set forth in claim 3,wherein said third means includes first counter for incrementing countervalue by a value proportional to a period of time to maintain said firstanti-skid brake control valve at said first mode position and a secondcounter for increasing counter value by a value proportional to a periodof time to maintain said second anti-skid brake control valve at saidfirst mode position, and detector for detecting synchronously increasingof braking pressure in said first and second wheel cylinders bycomparing a difference of said counter values of said first and secondcounters with a given value.
 5. An anti-skid brake control system as setforth in claim 4, which further comprises first arithmetic means forderiving first wheel acceleration based on said first wheel speedindicative signal to produce a first wheel acceleration indicativesignal, a second arithmetic circuit for deriving second wheelacceleration based on said second wheel speed indicative signal value toproduce a second wheel acceleration indicative signal, a first peak holdcircuit for holding peak value of said first wheel accelerationindicative signal in each skid cycle, and a second peak hold circuit forholding peak value of said second wheel speed indicative signal in eachskid cycle.
 6. An anti-skid brake control system as set forth in claim5, wherein said third means further includes a first comparator forcomparing said first counter value with a first reference value derivedbased on said peak value for producing a first comparator signal whensaid first counter value is greater than or equal to said firstreference value, and a second comparator for comparing said secondcounter value with a second reference value derived based on said peakvalue for producing a second comparator signal when said second countervalue is greater than or equal to said second reference value, and saidthird means is responsive to said first and second comparator signalsunder the presence of said detector signal to control operation of saidfirst and second anti-skid brake control valve with said delay time. 7.An anti-skid brake control system for an automatic vehicle comprising:ahydraulic brake circuit including a plurality of wheel cylinders forestablishing braking pressure for decelerating vehicular wheels; firstand second anti-skid brake control valves, each associated with at leastone of first and second wheel cylinders associated with first and secondwheels for adjusting braking pressure therein, each of said anti-skidbrake control valves being operable to increase braking pressure in theassociated wheel cylinder in a first mode position, and to decrease thebraking pressure in the associated wheel cylinder in a second modeposition; first sensor for monitoring rotation speed of said first wheelfor producing a first wheel speed indicative signal; second sensor formonitoring rotation speed of said second wheel for producing a secondwheel speed indicative signal; first means for deriving a first controlsignal for controlling said first anti-skid brake control valve based onsaid first wheel speed indicative signal between said first and secondmode positions; second means for deriving a second control signal forcontrolling said second anti-skid brake control valve based on saidsecond wheel speed indicative signal between said first and second modepositions; and third means for detecting synchronous increasing ofbraking pressure in said first and second wheel cylinders to causesynchronous locking of said first and second wheels, and operating saidfirst and second anti-skid brake control valves into said first modeposition so as to increase braking pressure in said first and secondwheel cylinders at mutually different timings and thus preventingsynchronous locking of said first and second wheels.
 8. An anti-skidbrake control system for an automotive vehicle comprising:a hydraulicbrake circuit including a plurality of wheel cylinders for establishingbraking pressure for decelerating vehicular wheels; first and secondanti-skid brake control valves, each associated with at least one offirst and second wheel cylinders associated with first and second wheelsfor adjusting braking pressure therein, each of said anti-skid brakecontrol valves being operable to increase braking pressure in theassociated wheel cylinder in a first mode position, and to decrease thebraking pressure in the associated wheel cylinder in a second modeposition; first sensor for monitoring rotation speed of said first wheelfor producing a first wheel speed indicative signal; second sensor formonitoring rotation speed of said second wheel for producing a secondwheel speed indicative signal; first means for deriving a first controlsignal for controlling said first anti-skid brake control valve based onsaid first wheel speed indicative signal between said first and secondmode positions; second means for deriving a second control signal forcontrolling said second anti-skid brake control valve based on saidsecond wheel speed indicative signal between said first and second modepositions; and third means for monitoring wheel speed variation rate onthe basis of said first and second wheel speed indicative signals todetect a wheel speed variation rate smaller than a predetermined value,said third means producing a third control signal for providing a givendelay time to one of said first and second means to operate anassociated one of said first and second anti-skid brake control valvesinto said first mode position so as to increase braking pressure in saidfirst and second wheel cylinders at different timings and thuspreventing synchronous locking of said first and second wheels.
 9. Ananti-skid brake control system for an automotive vehicle comprising:ahydraulic brake circuit including a plurality of wheel cylinders forestablishing braking pressure for decelerating vehicular wheels; firstand second anti-skid brake control valves, each associated with at leastone of first and second wheel cylinders associated with first and secondwheels for adjusting braking pressure therein, each of said anti-skidbrake control valves being operable to increase braking pressure in theassociated wheel cylinder in a first mode position, and to decrease thebraking pressure in the associated wheel cylinder in a second modeposition; first sensor for monitoring rotation speed of said first wheelfor producing a first wheel speed indicative signal; second sensor formonitoring rotation speed of said second wheel for producing a secondwheel speed indicative signal; first means for deriving a first controlsignal for controlling said first anti-skid brake control valve based onsaid first wheel speed indicative signal between said first and secondmode positions; second means for deriving a second control signal forcontrolling said second anti-skid brake control valve based on saidsecond wheel speed indicative signal between said first and second modepositions; and third means for monitoring wheel speed variation rate onthe basis of said first and second wheel speed indicative signals todetect a wheel speed variation rate smaller than a predetermined value,said third means operating said first and second anti-skid brake controlvalves into said first mode positions so as to increase braking pressurein said first and second wheel cylinders at mutually different timingsand thus preventing synchronous locking of said first and second wheels.10. An anti-skid brake control system for an automotive vehiclecomprising:a hydraulic brake circuit including a plurality of wheelcylinders for establishing braking pressure for decelerating vehicularwheels; first and second anti-skid brake control valves, each associatedwith at least one of first and second wheel cylinders associated withfirst and second wheels for adjusting braking pressure therein, each ofsaid anti-skid brake control valves being operable to increase brakingpressure in the associated wheel cylinder in a first mode position, andto decrease the braking pressure in the associated wheel cylinder in asecond mode position; first sensor for monitoring rotation speed of saidfirst wheel for producing a first wheel speed indicative signal; secondsensor for monitoring rotation speed of said second wheel for producinga second wheel speed indicative signal; first means for deriving a firstcontrol signal for controlling said first anti-skid brake control valvebased on said first wheel speed indicative signal between said first andsecond mode positions; second means for deriving a second control signalfor controlling said second anti-skid brake control valve based on saidsecond wheel speed indicative signal between said first and second modepositions; and third means for monitoring road surface friction on thebasis of said first and second wheel speed indicative signals to detecta road friction smaller than a predetermined value, said third meansproducing a third control signal for providing a given delay time to oneof said first and second means to operate an associated one of saidfirst and second anti-skid brake control valves into said first modeposition so as to increase braking pressure in said first and secondwheel cylinders at different timings and thus preventing synchronouslocking of said first and second wheels.
 11. An anti-skid brake controlsystem for an automotive vehicle comprising:a hydraulic brake circuitincluding a plurality of wheel cylinders for establishing brakingpressure for decelerating vehicular wheels; first and second anti-skidbrake control valves, each associated with at least one of first andsecond wheel cylinders associated with first and second wheels foradjusting braking pressure therein, each of said anti-skid brake controlvalves being operable to increase braking pressure in the associatedwheel cylinder in a first mode position, and to decrease the brakingpressure in the associated wheel cylinder in a second mode position;first sensor for monitoring rotation speed of said first wheel forproducing a first wheel speed indicative signal; second sensor formonitoring rotation speed of said second wheel for producing a secondwheel speed indicative signal; first means for deriving a first controlsignal for controlling said first anti-skid brake control valve based onsaid first wheel speed indicative signal between said first and secondmode positions; second means for deriving a second control signal forcontrolling said second anti-skid brake control valve based on saidsecond wheel speed indicative signal between said first and second modepositions; and third means for monitoring a road friction coefficient ofa road surface based on a wheel speed variation rate which is detectedon the basis of said first and second wheel speed indicative signals todetect a road friction coefficient smaller than a predetermined value,said third means operating said first and second anti-skid brake controlvalves into said first mode position so as to increase braking pressurein said first and second wheel cylinders at mutually different timingsand thus preventing synchronous locking of said first and second wheels.